PERC (Passivated Emitter Rear Contact) silicon solar cells are a special type of conventional crystalline silicon solar cells, characterized in that medium passivation layers exist both on the front surface and on the back surface of a solar cell. At present, reducing the cost of crystalline silicon is one of the goals of the photovoltaic industry full of increasingly fierce competition. Generally, making silicon wafers thinner is a development direction for silicon raw material cost reduction. Application of thinner silicon wafers is one of the trends in the future development of crystalline silicon solar cells. When the minority carrier diffusion length is larger than the silicon wafer thickness, the influence of the recombination rate on the back and front surfaces of the cell wafer on the photovoltaic conversion efficiency becomes more important. Improving the quality of surface passivation and decreasing the recombination rate have become the main methods to improve the efficiency of solar cells. To fabricate PERC cells, laser technology is used to notch on the back surface medium layer, so as to bare filiform or punctiform silicon substrates. The passivation film not only has an antireflection effect and increases the red light response, but also reduce the charge carrier recombination at the back surface. The photoelectric conversion efficiency of the solar cells with passivation films can improved 1.0-1.5%. Therefore, the back surface passivation structure is generally used in commercial crystalline silicon solar cells.
Based on the advantages of PERC cells, point contact aluminum back field structure has been paid more and more attention by global solar cell manufacturers, and its industrialization trend has become obvious. Compared with aluminum pastes for conventional aluminum back surface field cells (‘conventional aluminum pastes’ for short), the aluminum pastes for point contact aluminum back field cells meet higher technical requirements. Conventional aluminum pastes cannot fill well the filiform or punctiform areas exposed in passivation film, cannot form good ohmic contact with silicon substrate after being sintered. Moreover, conventional aluminum pastes have a very strong erosion against the passivation film, which may cause serious damage to the back surface field passivation film. Therefore, it is necessary to develop an aluminum paste suitable for the point contact aluminum back surface field structures. However, during the laboratory research and development processes, it was found that a large number of cavities occurred in the area of the point contact aluminum back surface field after being sintered. These cavities hinder the formation of P+ layer in the aluminum back surface field, deteriorate the ohmic contact, and thus affect the performance of solar cells.
In order to solve the poor filling capacity of point contact back surface field aluminum pastes, and to reduce or eliminate these cavities, the invention provides a method by adding a nanosized aluminum-boron-antimony alloy powder which has a high activity. The existence of boron and antimony in the nanosized aluminum-boron-antimony alloy powder makes the glass powder has good wettability, and at the same time, makes the sintering window adjustable; Tetrabutyl titanate and zinc methacrylate are added simultaneously with nanosized aluminum boron antimony alloy powder. The softening point of glass powders is controlled by compounding of the raw materials. The addition of tetrabutyl titanate and zinc methacrylate makes the thermal stability of glass powders increase, makes the omhic contact become better, and effectively improve the fillibility at the point contact back surface field by the aluminum paste. The filling ratio is more than 90% with the use of the aluminum paste in the invention.
A method which can effectively eliminate the cavities in point contact aluminum back surface field in PERC silicon solar cells is disclosed in Chinese Patent CN 103219416A. A double deposition method is used. Firstly, an aluminum layer is deposited on the areas without back surface passivation film in a crystalline silicon solar cell, and aluminum back surface field is formed after being sintering. Secondly, an aluminum layer is deposited on the partial or entire back surface, and then a back surface metal electrode is formed under low temperature. However, this method is too complicated to apply to the existing production processes.
A special aluminum paste for point contact aluminum back field crystalline silicon solar cells is disclosed in Chines Patent CN 103545013A. Compared with conventional aluminum pastes, the invented aluminum paste has the advantages of good flowability, little damage to the passivation film, good compactness and uniformity. But the filling effect of the aluminum paste is not mentioned.
It is known that point contact aluminum back fields of PERC cells are prone to producing cavities. However, there have not been reports by patent documents at home and abroad on improving the paste filling ratio to more than 90%.